The present invention relates to novel compounds and methods of preparation thereof and, particularly, to silyl camptothecin derivatives or analogs and to methods of preparation of such silyl camptothecin analogs.
(20S)-Camptothecin (CPT, see below) and its derivatives are some of the most promising agents for the treatment of solid tumors by chemotherapy. See, for example, Wall, M. E. et al, J. Ethnopharmacol., 51, 239 (1996); Camptothecin: New Anticancer Agents; Potmesil, M. and Pinedo, H., Eds.; CRC, Boca Raton, Fla. (1995); Bonneterre, J., Bull. Canc., 82, 623 (1995); Sinha, D. K., Drugs, 49, 11 (1995). This natural alkaloid was first isolated in 1966 from the extract. of a Chinese plant, Camptotheca accuminata, by Wall. Wall, M. E. et al, J. Am. Chem. Soc., 88, 3888 (1966). As depicted below, camptothecin has a fused ring system generally comprising a pyrrolo[3,4-b]quinoline system (rings ABC) fused to a 2-pyridone ring (ring D), which, in turn, is fused to a lactone ring (ring E). 
Camptothecin belongs to the family of topoisomerase I poisons. See, for example, Froelich-Ammon, S. J. et al., J. Biol. Chem., 270, 21429 (1995). Research to date strongly suggests that this molecule acts by interfering with the unwinding of supercoiled DNA by the cellular enzyme topoisomerase I, an enzyme which is usually overexpressed in malignant cells. In the highly replicating cancer cells, this triggers a cascade of events leading to apoptosis and programmed death. See Slichenmyer, W. J. et al., J. Natl. Cancer Inst., 85, 271 (1993). Recent advances at the molecular pharmacology level are reviewed in Pommier, Y. et al., Proc. Natl. Acad. Sci. USA , 92, 8861 (1995).
Camptothecin""s initial clinical trials were limited by its poor solubility in physiologically compatible media. Moreover, early attempts to form a water-soluble sodium salt of camptothecin by opening the lactone ring with sodium hydroxide resulted in a compound having a poor antitumor activity. It was later reported that the closed lactone-form is an absolute requisite for antitumor activity. See Wani, M. C. et al., J. Med. Chem., 23, 554 (1980). More recently, structure-activity studies have identified analogous compounds with better solubility and better antitumor activity. For example, topotecan (TPT) and irinotecan (IRT) have recently been approved for sale in the United States, while GI-147211C is in late stage clinical trials. These analogs are effective against a variety of refractory solid tumors such as malignant melanoma, stomach, breast, ovarian, lung and colorectal cancers, and seem particularly promising for the treatment of slow-dividing cancer lines. See, for example, Kingsbury, W. D. et al., J. Med. Chem., 34, 98 (1991); Sawada, S. et al., Chem. Pharm. Bull., 39, 1446 (1991); Luzzio, M. J. et al., J. Ned. Chem., 38, 395 (1995); Abigerges, D. et al., J. Clin. Oncol., 13, 210 (1995). Furthermore, synergistic or additive effects have been observed in combination therapies with cisplatin, irradiation, or hyperthermia. See Fukuda, M. et al., Canc. Res., 56, 789 (1996); Goldwasser, F. et al., Clin. Canc. Res., 2, 687 (1996); Wang, D. S. et al., Biol. Pharm. Bull., 19, 354 (1996).
Although most research has focused on the development of water-soluble derivatives of camptothecin, new formulations, such as lipid-complexation, liposomal encapsulation, and wet milling technology have recently been developed. Such formulations result in new therapeutic opportunities for poorly water-soluble camptothecins. See Daoud, S. S. et al., Anti-Cancer Drugs, 6, 83 (1995); Merisko-Liversidge, E. et al., Pharm. Res., 13, 272 (1996); and Pantazis, P., Leukemia Res., 19, 775 (1995). An attractive feature of these formulations is their impact on drug biodistribution. Sugarman and coworkers have recently reported that while free camptothecin achieves the greatest concentration in the pulmonary parenchyma, lipid-complexed camptothecin has the highest concentration in the gastrointestinal tract. These results open new and interesting perspectives for the treatment of colon cancer. See Sugarman, S. M. et al., Canc. Chemother. Pharmacol., 37, 531 (1996). Another interesting aspect of using insoluble camptothecin analogs is that they are usually more active than their water-soluble congeners and seem less likely to create drug-induced resistance, probably because they are not substrates of the p-glycoprotein multi-drug transporter. See Pantazis, P., Clin. Canc. Res., 1, 1235 (1995).
In this context, new camptothecin analogs that combine good to excellent anti-tumor activities with different solubility and biodistribution profiles could play a crucial role in the therapeutic arsenal for the treatment of various types of cancers.
Given the proven beneficial biological activity of camptothecin and analogs thereof, it is desirable to develop additional camptothecin analogs and methods of preparation of camptothecin analogs.
The present invention provides generally a compound having the following formula (1): 
R1 and R2 are independently the same or different and are preferably hydrogen, an alkyl group, an alkenyl group, an alkynyl group, an alkoxyl group, an aryloxy group, an acyloxy group, xe2x80x94OC(O)ORd, wherein Rd is an alkyl group, a carbamoyloxy group, a halogen, a hydroxyl group, a nitro group, a cyano group, an azido group, a formyl group, a hydrazino group, an acyl group, an amino group, xe2x80x94SRc, wherein, Rc is hydrogen, an acyl group, an alkyl group, or an aryl group, or R1 and R2 together form a group of the formula xe2x80x94O(CH2)nOxe2x80x94 wherein n represents the integer 1 or 2.
R3 is preferably H, a halogen, a nitro group, an amino group, a hydroxyl group, or a cyano group. R2 and R3 can also together form a group of the formula xe2x80x94O(CH2)nOxe2x80x94 wherein n represents the integer 1 or 2.
R4 is preferably H, F, a C1-3 alkyl group, a C2-3 alkenyl group, a C2-3 alkynyl group, or a C1-3 alkoxyl group. R5 is preferably a C1-10 alkyl group. A preferred alkyl group is an ethyl group. Preferred substituted alkyl groups for R5 include an allyl group, a propargyl and a benzyl group.
R6, R7 and R8 preferably are independently (the same or different) a C1-10 alkyl group, a C2-10 alkenyl group, a C2-10 alkynyl group, or an aryl group. A preferred substituted alkyl group for R6, R7 and R8 is a xe2x80x94(CH2)NR9 group, wherein N is an integer within the range of 1 through 10 and R9 is a hydroxyl group, an alkoxyl group, an amino group, a halogen atom, a cyano group or a nitro group. Preferred amino groups for R9 include alkylamino groups and a dialkylamino groups.
The terms xe2x80x9calkylxe2x80x9d, xe2x80x9carylxe2x80x9d and other groups refer generally to both unsubstituted and substituted groups unless specified to the contrary. Unless otherwise specified, alkyl groups are preferably C1-C15 (that is, having 1 to 15 carbon atoms) alkyl groups, and more preferably C1-C10 alkyl groups, and can be branched or unbranched, acyclic or cyclic. The above definition of an alkyl group and other definitions apply also when the group is a substituent on another group (for example, an alkyl group as a substituent of an alkylamino group or a dialkylamino group). The term xe2x80x9carylxe2x80x9d refers to phenyl or napthyl. As used herein, the terms xe2x80x9chalogenxe2x80x9d or xe2x80x9chaloxe2x80x9d refer to fluoro, chloro, bromo and iodo.
The term xe2x80x9calkoxylxe2x80x9d refers to xe2x80x94ORd, wherein Rd is an alkyl group. The term xe2x80x9caryloxyxe2x80x9d refers to xe2x80x94ORe, wherein Re is an aryl group. The term acyl refers to xe2x80x94OCRf. The term xe2x80x9calkenylxe2x80x9d refers to an unsaturated radical with preferably 2-15 carbon atoms, more preferably with 3-10 carbon atoms (xe2x80x94Cxe2x95x90CHRg). The term xe2x80x9calkynylxe2x80x9d refers to an unsaturated radical preferably with 2-15 carbon atoms, more preferably with 3-10 carbon atoms (xe2x80x94Cxe2x89xa1CRh).
The groups set forth above, can be substituted with a wide variety of substituents to synthesize camptothecin analogs retaining activity. For example, alkyl groups may preferably be substituted with a group or groups including, but not limited to, a benzyl group, a phenyl group, an alkoxyl group, a hydroxyl group, an amino group (including, for example, free amino groups, alkylamino, dialkylamino groups and arylamino groups), an alkenyl group, an alkynyl group and an acyloxy group. In the case of amino groups (xe2x80x94NRaRb), Ra and Rb are preferably independently hydrogen, an acyl group, an alkyl group, or an aryl group. Acyl groups may preferably be substituted with (that is Rf is) an alkyl group, a haloalkyl group (for example, a perfluoroalkyl group), an alkoxyl group, an amino group and a hydroxyl group. Alkynyl groups and alkenyl groups may preferably be substituted with (that is, Rg and Rh are preferably) a group or groups including, but not limited to, an alkyl group, an alkoxyalkyl group, an amino alkyl group and a benzyl group.
The term xe2x80x9cacyloxyxe2x80x9d as used herein refers to the group 
The term xe2x80x9ccarbamoyloxyxe2x80x9d as used herein refers to the group 
Amino and hydroxyl groups may include protective groups as known in the art. Preferred protective groups for amino groups include tert-butyloxycarbonyl, formyl, acetyl, benzyl, p-methoxybenzyloxycarbonyl, trityl. Other suitable protecting groups as known to those skilled in the art are disclosed in Greene, T., Wuts, P. G. M., Protective Groups in Organic Synthesis, Wiley (1991), the disclosure of which is incorporated herein by reference.
In general, R1, R2, R3, R6, R7 and R8 are preferably not excessively bulky to maintain activity of the resultant camptothecin analog. Preferably, therefore, R1, R2, R3, R6, R7 and R8 independently have a molecular weight less than approximately 250. More preferably R1, R2, R3, R61 R7 and R8 independently have a molecular weight less than approximately 200.
Some of the camptothecin analogs of the present invention can be prepared for pharmaceutical use as salts with inorganic acids such as, but not limited to, hydrochloride, hydrobromide, sulfate, phosphate, and nitrate. The camptothecin analogs can also be prepared as salts with organic acids such as, but not limited to, acetate, tartrate, fumarate, succinate, citrate, methanesulfonate, p-toluenesulfonate, and stearate. Other acids can be used as intermediates in the preparation of the compounds of the present invention and their pharmaceutically acceptable salts.
For purification purposes, the E-ring (the lactone ring) may be opened with alkali metal such as, but not limited to, sodium hydroxide or calcium hydroxide, to form opened E-ring analogs of compounds of formula (1). The intermediates thus obtained are more soluble in water and may be purified to produce, after treatment with an acid, a purified form of the camptothecin analogs of the present invention.
The E-ring may also be modified to produce analogs of compounds of formula (1) with different solubility profiles in water or other solvents. Methods to achieve this goal include, but are not limited to, opening the E-ring with an water-soluble amino group or functionalyzing the hydroxyl group at position 20 of the E-ring with a water-soluble group such as a polyethylene glycol group. The analogs thus prepared act as pro-drugs. In other words, these analogs regenerate the compounds of formula (1) (with the closed E-ring structure) when administered to a living organism. See, Greenwald, R. B. et al., J. Med. Chem., 39, 1938 (1996).
The present invention also provides a method of treating a patient, which comprises administering a pharmaceutically effective amount of a compound of formula (1) or a pharmaceutically acceptable salt thereof. The compound may, for example, be administered to a patient afflicted with cancer and/or leukemia by any conventional route of administration, including, but not limited to, intravenously, intramuscularly, orally, subcutaneously, intratumorally, intradermally, and parenterally. The pharmaceutically effective amount or dosage is preferably between 0.01 to 60 mg of the compound of formula (1) per kg of body weight. More preferably, the pharmaceutically effective amount or dosage is preferably between 0.1 to 40 mg of the compound of formula (1) per kg of body weight. In general, a pharmaceutically effective amount or dosage contains an amount of a compound of formula (1) effective to diaplay antileukemic and/or antitumor (anticancer) behavior. Pharmaceutical compositions containing as an active ingredient a compound of formula (1) or a pharmaceutically acceptable salt thereof in association with a pharmaceutically acceptable carrier or diluent are also within the scope of the present invention.
The present invention also provides a pharmaceutical composition comprising any of the compounds of formula (1) and a pharmaceutically acceptable carrier. The composition may contain between 0.1 mg and 500 mg of the compound of formula (1), and may be constituted into any form suitable for the mode of administration.